1. Field of the Invention
The present invention relates to an electrostatic display apparatus, and more particularly to an apparatus for displaying a pattern on a matrix type display board constituted of many electrostatically operated display units.
2. Prior Art
Firstly the principle of operation of an electrostatic display apparatus is described. The apparatus comprises a display board which is constituted of many electrostatic display units arranged along the length and breadth so as to form a display matrix. Each of the electrostatic display units is made up of a pair of electrodes: one is fixed and the other is movable. The fixed electrode is coated with a dielectric substance having a particular color. On the other hand the movable electrode is, for instance, made of a metal-coated plastic thin film so as to be bent over the fixed electrode by an electrostatic force produced when a high-tension voltage is imposed between both the electrodes. The movable electrode bent over the fixed electrode covers its surface to change the seeming color of the fixed electrode, that is, the appearance of the display unit is changed. Therefore, the display board can be made to display a predetermined pattern by selecting the distribution of high voltage supplied to the electrostatic display units.
An example of such an electrostatic display unit is shown perspectively in FIG. 1 and cross-sectionally in FIG. 2, in which an electric circuit to supply voltage is also shown. In this example two electrode plates 1 and 2 constitute the fixed electrode, while an aluminum coated polyester or polycarbonate film 3 is the movable electrode. The upper portions 1C, 2C and lower portions 1A, 2A of the two electrode plates 1 and 2 are flat and set up opposite to each other in parallel, while the middle portions extrude inside forming semi-cylindrical prominences 1B and 2B. The film-like movable electrode 3 runs through a shim inserted in the narrowest clearance 4 made between the semi-cylindrical prominences 1B and 2B. The lower portion of the movable electrode 3 is fixed to an elecrode holder 6, which doubles as a terminal 14. The holder 6 of the movable electrode 3 is fixed between the lower portions 1A and 2A of the two electrode plates by means of a male and female spacers 5, 7 and bolts 9 and 8. Of course the spacers 5 and 7 are made of an insulating material. The inner surfaces of the electrode plates 1 and 2, at least the area above the narrowest space 4 between them, are coated with insulating paints having their respective particular colors different from each other. In addition to the above arrangement of the electrodes, an A.C. voltage is supplied, as is shown in FIG. 2, between the movable electrode 3 (via the terminal 14) and the electrode plates 1 and 2 (via the terminal 12 and 13) from a voltage source 10, with the polarity of the movable electrode 3 changed by a switch 11.
In the above constitution of the electrostatic display unit, the movable electrode 3 is attracted, in accordance with its polarity, by either the electrode plate 1 or 2, and covers the inner surface of either of the electrode plates 1 and 2. Thus the appearance of the display unit can be changed between the two colors applied to the inside surfaces of the fixed electrodes.